Because it’s big. And it spins.
Seriously, it would have to be bigger than any space structure yet launched, it introduces extra stresses and failure points, and most importantly would make it difficult to dock or space walk. Every incoming ship would need to match the spin rate. Every space walk would tend to fling off the astronaut if something goes wrong.
Also, it may need to be bigger than you expect. There’s info somewhere on the internet estimating how big so the rotation doesn’t affect balance or cause nausea
Why doubt? That’s outstanding! As an astronaut, they want to be in space and have been trained for space. If I were an astronaut I’d be frustrated at piddly one week trips and want to be up there with the big boys taking a full shift off planet (not that I have any idea)